The present invention relates to waveform display instruments, and more particularly to an RGB display of a transcoded serial digital signal for the purpose of quantifying color gamut errors.
In a color display device, such as a shadow mask cathode ray tube (CRT) display, a video signal for display has three primary color components, such as red, green and blue (RGB). The display device is adjusted so that a minimum valid value of any one of the three components drives the corresponding electron gun to a minimum, or perceived off, condition and a maximum valid value drives the corresponding electron gun to maximum brightness. These valid values typically range between zero and 0.7 volts, and may be normalized arbitrarily to be between zero and 1.0. The primary color components (RGB) generally are derived from encoded luminance and color difference components (Y, P.sub.b, P.sub.r) using a resistive network. The encoded components are in turn derived from a composite video signal, such as a signal in accordance with the analog NTSC or PAL formats or the serial digital CCIR-601 format. In a digital suite digital is commonly used to reduce the number of wires required to route the video signal from one piece of equipment to another.
For many years the only significant source of a video signal was a video camera that provided the primary color components RGB. The RGB was then encoded into the color component signals Y, P.sub.b, P.sub.r and combined to produce the composite video signal in NTSC or PAL formats. However it is now common to generate the composite video signals from other sources than the primary color components RGB. Such sources, such as television graphics systems, may generate signals directly in the encoded color component domain. Also these encoded color components are the form in which video signal processing commonly is performed. The result is that when these encoded color components are transformed into the primary color component domain, the range or gamut of the individual primary colors may exceed the voltage range of zero to 0.7 volts. Values of the primary colors that are outside of the gamut may not be faithfully reproduced on the color display device, resulting in color distortion on the display.
One method of detecting this out-of-gamut condition, or gamut error, is disclosed in U.S. Pat. No. 4,707,727 issued Nov. 17, 1987 to Bruce J. Penney entitled "Apparatus for Providing an Indication That a Color Represented by a Y, R-Y, B-Y Color Television Signal is Validly Reproducible on an RGB Color Display Device" and incorporated into the WFM-300 Component Television Waveform Monitor manufactured by Tektronix, Inc. of Wilsonville, Oreg. The encoded color components are transformed into the primary color components and input to two sets of comparators, one set having as a reference level the maximum value and the other set having as a reference level the minimum value. The outputs of the comparators are input to an OR gate so that, if any one of the primary color components are above or below the maximum or minimum values, a gamut error signal is produced. The 500 Series Waveform Monitors/Vectorscopes manufactured by Magni Systems, Inc. of Beaverton, Oreg. provides a separate output for each comparator for display so that the primary color component and direction (plus or minus) of the gamut error is displayed.
Although the WFM-300 does display RGB component signals when the input is in that form, the WFM-300 does not display RGB component signals after being transformed from a video signal input in the encoded color component format. The WFM-300 does provide RGB output for a television monitor regardless of the input format. The problem is further compounded when the input video signal is in the CCIR-601 serial digital format. In order to display the digital input video signal, it is first transcoded into the encoded color component format. Once in the encoded color component format the video signal may be displayed as in the WFM-300.
What is desired is an RGB display of a transcoded serial digital video signal from which quantifiable gamut error may be derived.